CDs or DVDs as optical recording media are widely used because of their cost, compactness, stability and so on. When information recorded in these media is played, or when information is recorded in these media, optical pickup devices are used. A quarter-wave plate is used for optical pickup devices to convert linearly polarized laser light into circularly polarized laser light so as to irradiate it to an optical disc.
The quarter-wave plate is an optical element that modulates a phase by 90 degrees with using birefringence and functions to convert incoming linearly polarized light into circularly polarized light, or convert circularly polarized light into linearly polarized light to be emitted. For example, when a quarter-wave plate is formed using birefringence of quartz crystal, a refractive index of an ordinary ray and a refractive index of an extraordinary ray of quartz crystal are “no” and “ne” respectively, and a thickness of a quartz crystal substrate is “t”. A phase difference Γ between the ordinary ray and the extraordinary ray when light having a wavelength λ transmits through the quarter-wave plate is provided by Γ=2π/λ·(ne−no)·t. The phase difference Γ depends on the wavelength λ.
A broadband wave plate whose phase difference is nearly constant in a wavelength band of visible light is disclosed in Patent Document 1. As shown in FIG. 8(a), a quarter-wave plate 40 is composed of a half-wave plate 41, an adhesive 42, and a quarter-wave plate 43. As shown in FIG. 8(b), a stretching axis of the half-wave plate 41 is arranged in a direction of −15 degrees while a stretching axis of the quarter-wave plate 43 is arranged in a direction of −75 degrees against a polarizing direction of linearly polarized light entering into the quarter-wave plate 40. Note that the angles of the stretching axes are stated as angles in which the right side from a y axis on a yz plane is a positive sense. It is disclosed that these half-wave plate 41 and quarter-wave plate 43 are stretched high-molecular-weight films made of polycarbonate, and the quarter-wave plate 40 functions as a nearly perfect quarter-wave plate without depending upon a wavelength in a range of visible light (from 400 nm to 700 nm). Further, a function of the quarter-wave plate 40 is explained with the Poincare sphere.
On the other hand, a quarter-wave plate using a high order mode is disclosed in Patent Document 2. FIG. 9(a) is a plan view of a quarter-wave plate 50 from an incident direction while FIG. 9(b) is a schematic perspective view thereof. It is disclosed that the quarter-wave plate 50 includes a quartz crystal plate 51 having a phase difference of 1695 degrees (a fourth mode 255 degrees) with respect to a wavelength of 785 nm (a wavelength of laser light used for CDs) and an optical axis orientation (an angle made by a polarized direction of linearly polarized light entering into a wave plate and an optical axis) θ1 of 25.5 degrees (An anticlockwise direction is positive here), and a quartz crystal plate 52 having a phase difference of 850 degrees (a second mode 130 degrees) with respect to a wavelength of 785 nm and an optical axis orientation of 79.8 degrees bonded together so that an intersection angle θ3 of each of optical axes 53 and 54 intersected is 54.3 degrees, functioning as a quarter-wave plate in a wavelength band of 655 nm (a wavelength of laser light used for DVDs) and a band of 785 nm as a whole.
A function of the quarter-wave plate 50 is roughly explained using the Poincare sphere. However, detailed analysis is indicated with a following equation using Mueller matrices A1 and A2 respectively for the quartz crystal plates 51 and 52, and Stokes vectors T and S showing respective polarized states of incident and outgoing light.S=A2·A1·T  (1)
A phase difference of the quarter-wave plate 50 can be obtained from a component of the Stokes vector S.
It is disclosed that when a phase difference and an optic axis orientation of each quartz crystal plate with respect to the wavelength of 785 nm are (δ1,θ1,δ2,θ2)=(1695°,25.5°,850°,79.8°), and each optical axis is set to be intersected at an angle of 54.3 degrees, a phase difference Γ of the laminated quarter-wave plate 50 is a phase difference of 270 degrees at a wavelength of 655 nm and a phase difference of 90 degrees at 785 nm.
Further, as a second embodiment, it is disclosed that a phase difference Γ of a quarter-wave plate including a quartz crystal plate having a phase difference of 1980 degrees (a fifth mode 180 degrees) with respect to a wavelength of 655 nm and an optic axis orientation of 14 degrees, and a quartz crystal plate having a phase difference of 990 degrees (a second mode 270 degrees) and an optic axis orientation of 72 degrees bonded together so as to intersect each optical axis at an angle of 58 degrees is a phase difference of 270 degrees at a wavelength of 655 nm and a phase difference of 90 degrees at 785 nm.    [Patent Document 1] Japanese Unexamined Patent Publication No. 10-68816    [Patent Document 2] No. WO2003/091768